In their efforts to meet the demands of today's competitive and worldwide oriented market structures, producers of paper pulp are confronted with stringent requirements for their supplied pulp product to be of the highest quality. For this reason, a standard paper pulp production process usually includes a step of pulp bleaching, considerably improving brightness values of raw pulp, stemming from a mechanical or chemical pulping process.
Typically, the raw pulp is subjected to a treatment with an oxidizing agent, such as a chlorine-containing compound, for example molecular chlorine or a hypochlorite-containing solution, ozone, or a peroxide-containing compound, for example hydrogen peroxide. Stricter environmental demands, however, have increased the need not only to reduce chlorine charges in the bleaching of pulps, but also to replace altogether the chlorine base chemicals from bleaching sequences plants. An option to be used instead of chlorine bleaching is for example pulp bleaching using a peroxide compound as oxidizing agent. The peroxide process is often economically more beneficial, as well as technically easier to realize, when for compared to other processes, such as ozone bleaching. The art of peroxide bleaching involves the use of a bleach composition containing a peroxide compound as oxidant, such as hydrogen peroxide, an alkali source commonly furnished by sodium hydroxide, sodium silicate as buffer as well as hydrogen peroxide stabilizer, and other auxiliary chemical reagents such as magnesium compounds and chelating agents. The process is usually performed at elevated temperature for a given period of time.
The conventional hydrogen peroxide bleaching process, however, is affected by critical limitations in terms of efficiency and selectivity, which are not to be overcome in a straightforward manner by further fine-tuning the process parameters. One of these aspects concerns bleaching efficiency. Hydrogen peroxide bleaching has a typical profile of brightness development in dependence on bleach chemical consumption, in which while increasing a hydrogen peroxide dosage coupled with an adequate amount of alkali, an initial gain in brightness is rapid, then slows down, and finally reaches a plateau. A bleaching process occurring under conditions corresponding to the slow-down phase or to the plateau phase, respectively, is characterized by low efficiency and low selectivity. In this bleaching phase, brightness gain magnitude is smaller while consuming larger amounts of bleach chemicals, and at the same time causing an accelerated drop in pulp yield. This phenomenon is referred to as “brightness ceiling”. The brightness level of such a brightness ceiling varies, depending on lignocellulosic species contained within a pulp substrate.
A second aspect associated with peroxide bleaching is bleaching selectivity. In most cases, bleaching is accompanied by pulp yield loss. The yield loss reflects dissolution of components from the pulp fibres into a water-containing phase of a bleaching medium. Typically, the degree of yield loss is much more pronounced when pulp is bleached to high brightness.
It would be highly advantageous to have at one's disposal a method exploiting all beneficial aspects of hydrogen peroxide bleaching of pulp, and allowing for obtaining a high degree of pulp brightness, while at the same time keeping the pulp yield loss at a low value.